From the state of the art numerous printing systems are known that, especially in industrial environments, often include some kind of automatic detection mechanism for detecting such obstructions and thus being able to properly protect the vulnerable and expensive printheads during operation.
For example, single pass printing systems are known in which a plurality of print modules with print heads are placed next to each other such that they together cover a printing width in a transverse direction. During a printing operation, a substrate is moved stepwise underneath the print heads while being printed by them. In front of the print modules, a detection mechanism is placed. The detection mechanism comprises a plurality of light transmitters and a plurality of light sensors that are positioned at opposing transverse sides of the substrate. The light transmitters are positioned and directed such that they are able to transmit light beams closely above the substrate during operation. When one or more of the light beams gets blocked by an obstruction that projects upwardly from the substrate, like for example a crumple in the substrate, then this causes the light sensors to sense shadow. A detection signal is then sent out to a control unit that immediately stops the printing operation. The substrate no longer gets moved further towards the printheads, and thus the obstruction is prevented from running against one or more of the printheads.
A disadvantage with this is that the functioning of the detection mechanism leaves to be improved. Sometimes it occurs that the printing operation is stopped although no obstruction is present on top of the substrate. This may be caused by temperature changes, in which expansion of the light transmitters or sensors or their mounting means may occur and because of which a breaking index above the substrate may change due to air temperature changes. This may lead to the light beams getting somewhat redirected and thus getting oriented to close above the substrate with the risk of running partly against the substrate, causing shadow on the light sensors and unnecessarily stopping the printing operation. Even worse it may also occur that because of temperature changes the light beams get oriented too far above the substrate. This may lead to a situation in which the light beams pass over an obstruction without being interrupted by it at all. The printing operation then does not get stopped at all with the effect that the printheads may get damaged by such an undetected obstruction. Another disadvantage is that it is difficult to accurately adjust the light beams and sensors relative to each other and relative to the substrate, in particular when large printing widths of several meters need to be covered. Yet another disadvantage is that the detection mechanism requires free space at the opposing transverse sides of the substrate at the level of the substrate itself.
Scanning printing systems are also known to be equipped with a detection mechanism. In such a scanning printing system a plurality of print modules are placed in a carriage that is movable back and forth in a transverse direction over a substrate during a printing operation. Mechanically operable detection mechanism may be connected to opposing carriage sides that face the transverse direction. Those detection mechanisms thus move along with the carriage back and forth in the transverse direction during a printing operation. Because of that it is not possible to use light transmitters that get positioned so close to the substrate that they are able to transmit their light beams closely above the substrate during operation. Instead, an elongate plate is known to be used as detection element. The plate extends over the entire printing width and is hingedly connected to its respective side of the carriage while hanging downwards at a height above the substrate that is smaller than the spacing between the printheads and the substrate. End switches are positioned adjacent to the plate such that a swinging movement of the plate operates the switches. Such a swinging movement of the plate can be caused by the plate running against an obstruction on top of the substrate. A detection signal then is sent out to a control unit that immediately stops the printing operation. The carriage does not get moved further in the transverse direction over the substrate, and thus the obstruction is prevented from running against one or more of the printheads. Instead of end switches positioned adjacent the plate, it is also possible to have the plate itself connected to movable parts of switches, or to provide photo electric sensors adjacent the plate that get activated by a swinging movement thereof.
A disadvantage with this is that the high acceleration and deceleration forces that get exerted on the carriage during its high speed back and forth movements over the substrate, may accidentally induce a swinging movement of the plate and thus stop a printing operation without an obstruction being actually detected. On the other hand it has also occurred that the plate, instead of being pushed away by an obstruction, started to press the obstruction into the substrate and thus damage the substrate, instead of sending out a detection signal towards the control unit that an emergency stop needed to be performed. With this it is noted that all kinds of measures have been taken in order to decrease the weight of the plate. This however increased the risk of the plate starting to buckle when bumping against an obstruction, which buckling in turn stiffened the plate and thus increased the risk for malfunctioning of the detection mechanism.